This invention relates to the field of micro-scale flexural plate wave devices and more particularly to the active control of a flexural plate wave device to continuously propagate traveling waves therein, and to pump fluids by mounting the device with a fluid channel. The term active control denotes forcing and controlling a desired response in an excited device.
Micro-scale devices (e.g., with part dimensions around 2000 .mu.m in length or smaller, and less than 1 or 2 .mu.m in thickness) such as micro-chips, micro-electro-mechanical devices, and micro-fluid circuits often require micro-scale fluid pumps to cool or drive them. Micro-pumps can be used to drive micro-cooling systems for elimination of detrimental heat in circuitry, to manipulate micro-scale objects, and to power micro-fluid systems as an alternative to micro-electronics.
Wave Motion
The excitation of bounded objects such as plates and membranes can produce resultant wave motion within the excited object. Whenever excitation matches the inherent natural frequency of an object: a standing wave develops, the object is said to be in resonance, and the object exhibits an enhanced response.
When an object is unbounded, or wave motion is removed from a boundary, waves propagate. Particles within the object oscillate in the same manner but with different phase to produce directional traveling waves. At boundaries, wave motion in an object may be reflected. A standing wave is composed of traveling waves reverberating between boundaries.
Existing Fluid Pumps And Micromotors
Mechanical pumps are used successfully at the larger macro-scale level in conventional applications to raise, transfer, or compress fluids. Conventional macro- scale mechanical pumps consist of diaphragms, valves, vibrating membranes, and other moving parts that require clearances between those moving parts. At the macro-scale, the required clearances are only a fraction of the size of the manufactured part. As the pump size is driven down to the micro-scale and even smaller, the clearances become much larger in comparison to the manufactured part. While manufacturing tolerances are extremely small, the tolerances are fixed by the fabrication methods and are not likely to be reduced indefinitely for increasingly smaller manufactured parts. Mechanical pumps, therefore, can be difficult to scale down to micro-scale, and problems can occur with clearances between moving parts; thus, mechanical pumps are not generally used as micro-pumps.
Peristaltic pumps have been proposed as an alternative to mechanical pumps. Peristaltic pumping is a form of fluid transport that occurs when progressive contraction or expansion propagates along the length of a distensible tube containing a liquid. Hartley, U.S. Pat. No. 5,705,018 (1998), describes a peristaltic pump in a channel, where sequential application of voltage generates electrostatic fields which sequentially excite a series of conductive strips lining the channel, which in turn successively pull an overlying flexible conductive membrane into the channel to achieve peristaltic pumping action. Electrostatic peristaltic pumping works for sequential excitation of the strips lining a membrane-enclosed channel, and thus progressive contraction and expansion (pulsating) of the channel. Electrostatic peristaltic pumps, however, can be difficult to fabricate and require a membrane-enclosed channel and an additional timing mechanism (oscillator clocking frequency) to provide a progressive rate of strip excitation. Further, Hartley's peristaltic pump only functioned with electrically nonconductive fluids. Magnetic embodiments for electrically conductive fluids would be more complex, require significantly greater amounts of power, and function over a more restrictive temperature range. See Hartley.
Piezoelectric pumps have been proposed as an alternative to mechanical pumps. Valveless fluid pumps can use millimeter-scale diffuser/nozzles with oscillating pump diaphragms, driven by piezoelectric discs. See Erik Stemme and Goran Stemme, Electronics Cooling Technical Brief: "A Valveless Fluid Pump for Electronics Cooling," January 1996, retrieved from the Internet. Diffusers are gradually expanding flow channels with flow resistance differences at the nozzle and diffuser ends, and are used to raise static pressure. Movement in a silicon and glass pump chamber, excited by piezoelectric discs, forces fluid through the diffuser/nozzles. See Anders Olsson et al., "Valve-less Fluid Micro-pump," KTH, Instrumentation Laboratory, S3, August 1997, retrieved from the Internet; Anders Olsson, Annual Report 95-96: Signals, Sensors and Systems, KTH, "Valve-less Diffuser Pumps for Liquids: Abstract," retrieved from the Internet. The piezoelectric pump works for pumping fluid using finite-sized fluid chambers and an oscillating diaphragm (pulsating fluid), and piezoelectric materials work for exciting and detecting acoustic waves. Piezoelectric materials' processing and fabrication, however, can be difficult due to piezoelectric materials incompatibility with a silicon processing line.
Micromotors have been used to move micro-objects on a membrane in a gaseous or vacuum environment. White, U.S. Pat. No. 5,006,749 (1991), describes a micromotor device for linear movement of one or more microelements on a membrane, with control maintained using a linear position sensor including a circuit for producing feedback control signals. The micromotor device works for moving miniature mechanical parts using ultrasonic waves and feedback control signals, but has not been applied to fluid motion.
Flexural Plate Wave Devices
Flexural plate wave devices represent a relatively new technology that shows promise for micro-scale application. Conventional flexural plate wave devices typically use interdigital transducers patterned on piezoelectric material to excite and detect acoustic waves in a composite, thin membrane. See Wenzel and White, "A Multisensor Employing an Ultrasonic Lamb-Wave Oscillator," IEEE Transactions on Electron Devices, Vol. 35, No. 6, pp. 735-743, June 1988. Wenzel and White describe a sensitive, composite silicon/piezoelectric device to excite and detect oscillating waves in a thin membrane on a Lamb-wave oscillator sensor. Wenzel and White consider propagation of a lowest order antisymmetric mode, whose wave velocity decreases to zero as the plate is made vanishingly thin. The confinement of acoustic energy in the thin membrane can make excited wave velocities that are extremely sensitive to surface perturbations such as mass accumulation or membrane tension, giving a sensitivity characteristic of flexural plate wave devices that can be useful in sensing applications. Wenzel and White's composite silicon/piezoelectric Lamb-wave device excites oscillating waves in a device for use as a sensor. A composite Lamb-wave device can be difficult to fabricate, however, and there is no teaching of using the device for anything other than an oscillator sensor.
In a magnetically-excited flexural plate wave device used as a resonator, the piezoelectric layer in the conventional composite membrane is eliminated to simplify device fabrication and integration with control electronics. See Martin et al., "Magnetically-Excited Flexural Plate Wave Resonator," Proceedings of the IEEE International Frequency Control Symposium, pp. 25-31, May 28-30, 1997, hereafter referred to as Martin'97. Magnetically-excited flexural plate wave devices can be manufactured using lithographic methods and bulk micromachining to construct a free-standing membrane on a silicon substrate. See Butler et al., "Magnetically-Excited Flexural Plate Wave Device," Transducers '97, International Conference on Solid-State Sensors and Actuators, Jun. 16-19, 1997. A flexural plate wave resonator can be used to produce resonant standing waves in a membrane, where the generated waves resonate between membrane-plate boundaries. Martin'97 presents two resonator models: a model that characterizes the impedance for a one-port device and a model that characterizes the transmission response for a two-port device. Waves are reflected by the edges to get a 2D resonant standing wave, or eigenmode, of a rectangular membrane. As an example, a flexural plate wave resonator device is disclosed as suitable for sensing environmental changes that affect membrane tension, such as temperature, differential pressure, and strain. Since the membrane is sensitive to surface mass accumulation, Martin'97 suggests that the addition of a chemically-sensitive film to a flexural plate wave device can enable sensitive chemical detection. The flexural plate wave resonator excites resonant standing waves in a device for use as a sensor, but there is no teaching of using it for propagating traveling waves for a fluid pump.
Schneider et al., U.S. Pat. No. 5,706,840 (1998), describes the use of flexural plate wave sensors in a precision cleaning apparatus. Spates et al., U.S. Pat. No. 5,661,233 (1997), describes an acoustic-wave sensor apparatus for analyzing a petroleum-based composition.
Moroney discloses a rotary motion pump with a flexural plate wave device. See Moroney et al., "Fluid Motion Produced by Ultrasonic Lamb Waves," IEEE Ultrasonics Symposium, Dec. 4-7, 1990. Moroney generated waves with pulsed excitation, and consequently small streaming motion. Moroney used flexural plate waves in microfabricated thin membranes, with a piezoelectric layer, to pump fluids in a rotary motion and to transport solids. Moroney generated a rotary motion on a Lamb-wave membrane device, where acoustic waves produced eliptical fluid circulation in an etched well, within a silicon layer, during a pulsed excitation. Moroney achieved rotary pumping with small streaming motion from pulsed excitations, but Moroney's device was not suitable for pumping fluid for continuous linear fluid flow through a channel.
Accordingly, there is an unmet need for a fluid pump at the micro-scale, that can produce continuous linear fluid flow through a channel, and can be made with simplified fabrication methods compatible with conventional silicon micro-machining technology.